While automatic block signal systems using wayside signals provide the primary control for railway vehicle operation, it is often desirable to have on-board signals to show track operating conditions. On-board, or cab signals, are particularly useful where rain, fog, or other environmental conditions make it difficult to see the wayside signal aspect. In addition, cab based signal displays permit a railway vehicle operator to monitor changing track conditions after the train has entered a block. Without cab signaling the train may only be permitted to proceed at a restricted speed, even if the block has now been cleared.
Cab signaling is well-known and has been used for many years with a transmitter applying a signal to the rails, and a railway vehicle mounted receiver inductively receiving the coded signal through two receiver coils mounted on the locomotive ahead of the leading wheels. The rail current between the transmitter and the leading axle is inductively sensed by the railway vehicle receiver and the appropriate signal is displayed in the vehicle cab.
When a train crosses the joints at the entering end of an unoccupied track circuit, its cab signal receiver will begin to sense the coded cab signal current in the rails immediately ahead of the leading axle. As the train proceeds through the track circuit, the level of this signal gets progressively higher as the rail impedance between the signal source and the train decreases. In track circuits the rail current can be as high as 20 amperes when the train reaches the leaving end, whereas the amount required to energize the cab receiver may be as low as 1.3 amperes. While the rail current is being sensed in advance of the leading axle, a certain amount of the track current that carries the cab signal is shunted through the railway vehicle wheel and axle assemblies, often referred to as the train shunt. If the impedance of the train shunt is above zero, even by as little as a few hundredths of an ohm, enough cab signal rail current may bypass the train to cause pickup of the cab signals by the receiver of a following train. This bypass cab signal current, referred to as runby, can, if sufficiently large, cause a second or following train to erroneously detect the clear signal intended for the lead train. Because the rail impedance and the ballast between the trains act to reduce the level of current reaching the following train, the problem of bypass current is particularly bothersome when the following train is in relatively close proximity to the lead train. In this condition, a substantial portion of the bypass current from the lead train is available to be sensed by the following train, and is highly undesirable.